"In the beginning – like when we did the jive – there were times we would run through the dance and after we were done I would literally just lay on the ground," Riley tells PEOPLE about her six-hour rehearsals with dance pro Derek Hough.
Riley entered the ABC dance show unsure of how she would stack up against the competition, but she has soared through the first half of season 17, often with near-perfect scores from the judges.
"I am genuinely shocked I am doing as well as I am," she says. "But I'm the type of person that if I feel something is going to defeat me, I am going to want to do it [even more]."
With such confidence, Riley has not only given herself the strength to reach the top seven but has also inspired women across the country to take their first dance and exercise classes.
"It is awesome that people are so touched by me just dancing on a television show," says Riley. "This is just something I did for fun, and it is helping a lot of people."
The dancing is helping her, as well. As a result of her lengthy rehearsals, Riley is noticing her clothes are fitting differently and her arms getting smaller. Riley says weight loss is not something she is focusing on – but she does enjoy shopping more now.
"There are a lot of plus-size [fashion] lines, but there really is not a lot to choose from. And I'm not the type of person who likes to wear what everyone else is wearing," says Riley, who dropped two dress sizes after health issues caused her to change her diet last year. "Being a little smaller does open up your closet to new choices, but it has made my spending a little worse."
And while her wallet may be lighter, the pressure to outdo herself on the dance floor increases each week as she gets closer and closer to the finale – and a chance to take home the mirror-ball trophy.
"There are moves that frustrate me and tears well up in my eyes, but I am taking this experience and I'm going for it," she says. "I'm going to do every single thing ... and try not to pass out."
For more on Amber Riley's inspirational journey with Dancing with the Stars, pick up this week's issue of PEOPLE, on newsstands now